Thermotropic polyesters which demonstrate optical anistropy in the melted state are known. These products have been described in the literature on many occasions, for example: "Liquid Crystal Polymer", British Polymer Journal, p. 154 (December 1980); "Liquid Crystalline Aromatic Polyesters", Journal of Macromolecular Science Chemistry, p. 170 (1984); "Rigid Chain Polymers", Die Augewandte Makromolekulare Chemie, 109-120, p. 1 (1982); "Thermotropic Liquid Crystalline Polymers", Die Augenwandte Makromolekulare Chemie, 145/146, p. 231 (1986); "Liquid Cristalline Polymers: A Novel State of Material", Journal of Molecular Science Review, C26 (4) p. 551 (1986).
From these polymers, it is possible to obtain high toughness fusion fibers or molded products, for example by injection molding. The products have suitable characteristics of rigidity, hardness and toughness.
The polymers with the properties referred to above must also be easy to process, highly resistant to heat and oxidation. Finally, these polymers have a high HDT (Heat Distortion Temperature) and resistance to attack by solvents due to their crystallinity.
Polymers which can be obtained by polycondensation of terephthalic acid and hydroquinone (poly 1-4-phenylenterephtalate) melt at temperatures which are too high (more than 600.degree. C.) for them to be processed without decomposition of the polymer itself. One way used to lower the melting point is to use substitutes in the aromatic ring of the hydroquinone or the terephthalic acid or to modify the polymer by adding other rigid comonomers.
U.S. Pat. No. 4,159,365 describes formation of a polymer obtained by condensation of terephthalic acid and phenyl hydroquinone containing up to 10% of another aromatic or cycloaliphatic group capable of forming polyesters. However, of these groups, other substituted hydroquinones are not specifically mentioned.
U.S. Pat. No. 4,360,658 describes formation of copolyesters by condensation of terephthalic acid, phenyl hydroquinone and quantities of hydroquinone with a mole content of 25 to 50% of the total quantity of diols.
U.S. Pat. No. 4,238,600 describes preparation of polymers obtained from terephthalic acid, phenyl hydroquinone and quantities of t-butyl-hydroquinone with a mole content of 25 to 65% of the quantity of diols.
The polymers described in the latter patent however have a melting point of more than 340.degree. C., increasing with the level of the contents of the t-butyl-hydroquinone in the starting mixture.
U.S. Pat. No. 4,447,593 describes preparation of polymers of terephtalic acid with hydroquinones substituted by alkyl groups containing at least 5 atoms of carbon or by aralkyl groups containing at least 7 atoms of carbon, in addition to other hydroquinones substituted up to a maximum percentage of 20%.
Finally, U.S. Pat. No. 4,600,765 reports formation of polyesters by polycondensation of therphthalic acid, phenyl hydroquinane and (1-phenylethyl) hydroquinone obtaining liquid crystalline polymers having a melting point around 320.degree. C.
All the preceding references are hereby incorporated by reference.